A VPN is a kind of network disposed in a public network infrastructure. The whole VPN network is a logic network constructed upon a network platform provided by a public network service provider, and there is no end-to-end physical link as required in a traditional private network between any nodes of the VPV network. User data are transmitted through logic links. A VPN service is a kind of intelligent network services. Its main object is to define telephone users of a company or a team as one group, and allocate each user within this group with a short number which can be used inside the group, and users within this group can directly call one another by dialing the short number.
To resolve the problem of user's malicious overdrawing, it is feasible to adopt prepaid services. A prepaid service is an important service provided by an intelligent network. A prepaid user needs to deposit a certain amount of fee in his prepaid account before calling and communication fee will be deducted from the corresponding account real-timely. If the fee in the account is used up, a current call will be terminated and the corresponding user terminal cannot initiate a call until this account is recharged.